twa_run: better protection against empty cycles

Fixes #337.

* spot/twaalgos/emptiness.cc, spot/twaalgos/emptiness.hh: Here.
* tests/python/except.py: Test it.
* NEWS: Mention the issue.

NEWS

@@ -12,6 +12,9 @@ New in spot 2.5.1.dev (not yet released)
   - "autfilt --acceptance-is=Fin-less" no longer accept automata
     with "f" acceptance.
 
+  - twa_run methods will now diagnose cases where the cycle is
+    unexpectedly empty instead of segfaulting.
+
 New in spot 2.5.1 (2018-02-20)
 
   Library:

spot/twaalgos/emptiness.cc

@@ -282,6 +282,13 @@ namespace spot
     return os;
   }
 
+  void twa_run::ensure_non_empty_cycle(const char* where) const
+  {
+    if (cycle.empty())
+      throw std::runtime_error(std::string(where)
+                               + " expects a non-empty cycle");
+  }
+
   namespace
   {
     class shortest_path final: public bfs_steps
@@ -343,6 +350,7 @@ namespace spot
 
   twa_run_ptr twa_run::reduce() const
   {
+    ensure_non_empty_cycle("twa_run::reduce()");
     auto& a = aut;
     auto res = std::make_shared<twa_run>(a);
     state_set ss;
@@ -502,6 +510,7 @@ namespace spot
 
   bool twa_run::replay(std::ostream& os, bool debug) const
   {
+    ensure_non_empty_cycle("twa_run::replay()");
     const state* s = aut->get_init_state();
     int serial = 1;
     const twa_run::steps* l;
@@ -754,6 +763,7 @@ namespace spot
   twa_graph_ptr
   twa_run::as_twa(bool preserve_names) const
   {
+    ensure_non_empty_cycle("twa_run::as_twa()");
     auto d = aut->get_dict();
     auto res = make_twa_graph(d);
     res->copy_ap_of(aut);

spot/twaalgos/emptiness.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2011, 2013, 2014, 2015, 2016, 2017 Laboratoire de
+// Copyright (C) 2011, 2013, 2014, 2015, 2016, 2017, 2018 Laboratoire de
 // Recherche et Developpement de l'Epita (LRDE).
 // Copyright (C) 2004, 2005 Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -402,6 +402,15 @@ namespace spot
     twa_run(const twa_run& run);
     twa_run& operator=(const twa_run& run);
 
+    /// \brief Raise an exception of the cycle is empty.
+    ///
+    /// It is OK for a twa_run to have an empty cycle while the run is
+    /// being filled by some procedure.  But after that, we expect
+    /// cycles to be non-empty.  Calling this function will raise an
+    /// std::runtime_error if the cycle is empty, giving \a where
+    /// (usually the name of the calling function) as context.
+    void ensure_non_empty_cycle(const char* where) const;
+
     /// \brief Reduce an accepting run.
     ///
     /// Return a run which is still accepting for <code>aut</code>,

tests/python/except.py

@@ -59,3 +59,19 @@ try:
     si.determine_unknown_acceptance()
 except RuntimeError as e:
     assert "scc_info::determine_unknown_acceptance() does not supp" in str(e)
+
+r = spot.twa_run(aut)
+try:
+    a = r.as_twa()
+except RuntimeError as e:
+    assert "empty cycle" in str(e)
+
+try:
+    a = r.replay(spot.get_cout())
+except RuntimeError as e:
+    assert "empty cycle" in str(e)
+
+try:
+    a = r.reduce()
+except RuntimeError as e:
+    assert "empty cycle" in str(e)